A direct-fired steam generator usually comprises a system formed from three parts, namely, a burner head, a combustion chamber, and a straight, or elbow-forming, tubular mixing chamber. Except for the mixing chamber, U.S. Pat. No. 4,211,071 discloses such a steam generator. Considerable heat is generated in the burner head, combustion chamber and mixing chamber
While the patented structure includes a water jacket for cooling the length of the combustion chamber, the bottom wall, which contains a centrally located exit outlet for conveying steam and hot combustion gases, is not adequately cooled. A solution to this cooling problem is disclosed in U.S. Pat. No. 3,980,137, wherein a bottom combustion chamber wall is made of upper and lower sections constructed for being clamped together to form an annular passage for receiving cooling water. However, this solution is somewhat costly.
Both of the aforementioned patented structures introduce feed water into the combustion chamber by having it enter from the top of the water jacket through a small gap provided between the top wall of the combustion chamber and the inner wall of the water jacket. This feed water then runs down the inner surface of the inner wall. This water flow serves the purposes of providing secondary cooling to the combustion chamber, and of introducing water into the hot products of combustion so that it changes to steam while preventing water from coming in direct contact with the flame, such direct contact being undesirable since it would negatively affect combustion. While this may be a suitable way to introduce feed water into a static combustion chamber, it has been found that in a mobile application, such as when the steam generator is being used to generate steam to re-hydrate crop just before baling, for example, the terrain traversed by the generator carrying vehicle may result in the combustion chamber becoming tilted, which causes an uneven flow of feed water along the inner wall of the combustion chamber. The result of uneven flow is that a portion of the water prematurely flashed to steam in the combustion chamber. As water flashes to steam, the water leaves behind solid particles (mineral deposits) on the combustion chamber walls and the steam disrupts the flame. The mineral deposits build up over time and will cause water flow and heat transfer issues resulting in unacceptable steam generator system performance. In addition, when water flow is disrupted, hot spots can occur in some designs on the lower parts of the combustion chamber which are not cooled by the water-jacket. Yet another disadvantage of this design is the abrupt transition at the bottom wall of the combustion chamber to go from the diameter of the combustion chamber to the smaller diameter of the exit conduit. This abruptness causes turbulence which requires an increase in burner blower power to move the combined steam and combustion gases through the system. Available power for implements can be very limited, especially in older machines; therefore, a design with excessive power requirements has little practicality for use in some mobile applications.
The aforementioned drawbacks associated with the known design has been solved in part by another known system wherein the feed water is injected as a fine mist or spray into the bottom zone of the combustion chamber at the tip of the flame, but the problem remains that the flat bottom wall of the combustion chamber still becomes too hot due to the fact that hot combustion gases impact the wall and must abruptly move to the middle of it before exiting. In this known steam generator layout, the bottom of the combustion chamber and an end of an exit conduit were each provided with a flange and these flanges were clamped and sealed to opposite faces of a water injection ring penetrated by a radially extending feed water pipe terminating at a discharge nozzle located centrally within the ring so as to meter water into a zone at the bottom of the combustion chamber. However, the flanges were found to reach an unacceptable temperature in the neighborhood of 735° F.
The problem to be solved then is to find a way to reduce the operating temperature of the exterior surfaces of the combustion chamber and exit conduit, located in the region of the bottom of the combustion chamber, to an acceptable temperature.